[WebAssembly] Make stack pointer args inhibit tail calls

Summary:
Also make return calls terminator instructions so epilogues are
inserted before them rather than after them. Together, these changes
make WebAssembly's tail call optimization more stack-safe.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73943